The invention relates to a decoupling element of deformable material, such as rubber or elastomer, for a rotary drive device such as a pulley, a wheel, etc. in a power transmission system for a compressor, alternator, starter, or any other type of accessory that needs to be driven in rotation.
As shown in the diagrammatic longitudinal and cross-section views of FIGS. 1a and 1b, pulleys or other rotary coupling means generally interpose a portion A of annular shape made of rubber having a section that is rectangular or shaped, between a rim B and a central hub C. By deforming in shear, between a rest position K0 and a position K in rotation, this piece allows a certain amount of angular offset to occur, thereby performing its decoupling function, in particular as a frequency filter and damper between the exciter (arrow E) and the response (arrow R). FIG. 1c shows a cross section similar to that of FIG. 1a with a link insert or friction layer D. The link insert is described in U.S. Pat. No. 5,377,962 to Ochs et al.
The ring A is generally fixed via its inside and outside faces by overmolding or bonding to cylindrical supports made of metal or plastics material and forming respectively the inner hub and the outer rim. It is generally also appropriate to bond the ring to strength members which are themselves force-fitted between the rim and the hub.
That solution, as disclosed for example in patent EP 0 740 077 presents numerous drawbacks, and in particular:                bonding requires coating and molding operations that are unsatisfactory in terms of cost and harm to the environment due to the use of adhesives and solvents;        the inserts which are generally made of metal give rise to non-negligible extra cost;        overmolding implies an additional swaging step to release differential stresses by radial deformation, and thus also leads to extra cost;        force-fitting the inserts leads to stresses in the parts; and        the system cannot be disassembled.        
In other documents, for example patent EP 0 793 031, the ring has successive concave and convex undulations of essentially continuous curvature facing complementary shapes made on the hub and the rim. The undulations perform two functions, that of filtering without bonding and that of limiting torque. The concave and convex shapes allow the parts to slip relative to each other beyond a cutoff torque, and also to return to a driving configuration below the cutoff.
That type of solution leads to non-linearity in the torque exerted as angular offset increases, and thus to non-linear stiffness, which is harmful to the filtering function. To be effective filtering must be tuned to specific cutoff frequencies that differ depending on the structure of the system or the type of transmission that is to be provided. However since the system is non-linear, its resonant bandwidth is very broad and filtering is not performed correctly.